1. Field of the Invention
The present invention pertains to an impeller, a pressure generator using such an impeller, and a pressure support system and method using such a pressure generator to deliver a flow of breathing gas to a patient, and, in particular, to an improved impeller that enables a pressure of a flow of breathing gas delivered by a pressure support system to a patient to remain substantially constant despite variations of the output flow rate.
2. Description of the Related Art
Pressure support systems that provide a flow of gas to an airway of a patient at an elevated pressure to treat a medical disorder are well known. For example, it is known to use a continuous positive airway pressure (CPAP) device to supply a constant positive pressure to the airway of a patient throughout the patient""s respiratory cycle to treat obstructive sleep apnea (OSA) as well as other cardiopulmonary disorders. An example of a CPAP device is the REMstar(copyright) and Solo(copyright) family of devices manufactured and distributed by Respironics, Inc. of Pittsburgh, Pa.
It is also known to provide a positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient""s breathing cycle. A conventional life support ventilator, such as the Esprit(copyright) Ventilator, also manufactured by Respironics, is an example of a pressure support system in which the pressure of gas delivered to the patient varies so as to replace or supplement the patient""s own respiration. Another example of a pressure support device that provides xe2x80x9cbi-levelxe2x80x9d pressure support in which a lower pressure is delivered during the patient""s expiratory phase than during the inspiratory phase, is the BiPAP(copyright) family of devices manufactured and distributed by Respironics, Inc. It is further known to provide an auto-titration positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea or upper airway resistance. An example of a device that adjusts the pressure delivered to the patient based on whether or not the patient is snoring is the Virtuoso(copyright) CPAP family of devices manufactured and distributed by Respironics, Inc. An example of a pressure support device that actively tests the patient""s airway to determine whether obstruction, complete or partial, could occur and adjusts the pressure output to avoid this result is the Tranquility(copyright) Auto CPAP device, also manufactured and distributed by Respironics, Inc.
In providing a respiratory therapy to a patient, a pressure support system, such as CPAP, ventilator, bi-level, or auto-titration pressure support device, typically delivers a flow of a breathing gas, such as air, oxygen, or an oxygen mixture, to an airway of the patient at a desired pressure, at least during a portion of the patient""s respiratory cycle. A CPAP system, for example, delivers a constant pressure to the patient""s airway throughout the patient""s respiratory cycle. A typical pressure support system includes a source of gas, such as atmosphere, an oxygen tank or the like, or a combination thereof, a pressure generator with a pressure or flow control capability to generate and maintain a flow of breathing gas at the selected pressure, a patient circuit to deliver the flow of breathing gas to the patient, and a patient interface to communicate the flow of breathing gas with the patient""s airway. The patient circuit typically is a flexible conduit having a patient interface device attached thereto. The patient interface is any device, such as a nasal/oral mask, nasal cannula, trachea tube, hood or the like, for communicating the flow of breathing gas with the patient""s airway.
The pressure generator in conventional a pressure support system typically includes a motor driving a blower, which is an impeller within a housing, for placing the gas from the gas source under pressure relative to ambient atmosphere. In a conventional blower, the pressure output by the blower varies with the rate of flow in the patient circuit, assuming that the blower operates at a constant speed. For example, at a certain operating speed, the pressure in the patient circuit or patient interface decreases as the flow of gas in the patient circuit or at the patient interface increases. This occurs, for example, as the patient breathes into the patient circuit. For this reason, a conventional pressure support system typically includes a pressure/flow control system, which includes a sensor and a processor operating in a feedback fashion, to control and maintain the desired pressure or flow for the breathing gas delivered to the patient as the flow within the patient circuit fluctuates.
Various pressure/flow control techniques are known for controlling the flow of breathing gas output by the pressure generator and/or delivered to the patient so that the output pressure remains constant at the selected pressure despite variations in the flow of gas in the patient circuit or at the patient interface. In each of the conventional pressure/flow control techniques, either the pressure of gas in the patient circuit or at the patient interface, the flow of gas in the patient circuit, or a combination thereof, must be monitored, either directly or indirectly, for providing a feedback signal in the feedback pressure/flow control system.
In a first type of conventional pressure/flow control technique, during operation, the blower operates at substantially constant speed that is sufficient to deliver a pressure in excess of the selected pressure at to be delivered to the patient. The pressure output by the blower must be high enough above the selected pressure to be delivered to the patient""s airway to allow the selected pressure to be delivered to the patient even if a pressure drop associated with an increase of flow occurs in the patient circuit. This pressure/flow technique also requires an exhaust valve to bleed off excess pressure or flow from the patient circuit, for example, when the flow in the patient circuit is relatively low. The amount of exhaust flow is controlled by the controller based on the feedback signals from the pressure and/or flow sensor to provide the selected pressure to the patient.
In a second type of conventional pressure/flow control technique, the motor in the blower does not operate at a constant speed, but is sped up or slowed down as needed to maintain the desired pressure. The motor speed is controlled by the controller based on the feedback signals from the pressure and/or flow sensor to provide the selected pressure to the patient. It is also known to use a combination of motor speed control and exhaust flow control to regulate the pressure or flow of breathing gas delivered to the patient.
These conventional types of pressure/flow control techniques, however, have certain drawbacks. In the exhaust flow control technique, bleeding off excessive pressure or flow requires the impeller to deliver more pressure than needed, which is inherently inefficient. In the motor speed control technique, cycling the motor speed is also inefficient and, due to the inertia of the impeller, does not allow for relatively rapid changes in the output pressure or flow. In addition, repeated cycling of the blower speed decreases the motor life as well as the efficiencies of the motor. It can be appreciated that either of these types of pressure control systems require a relatively expensive pressure and/or flow sensor to provide the feedback signal, as well as a relatively complicated pressure/flow control system that operates based on this feedback signal.
Accordingly, it is the object of the present invention to provide a pressure support system and method of providing a flow of breathing gas to a patient using an improved pressure generator and impeller design that overcomes the aforementioned shortcomings of conventional pressure support systems. It is a further object of the present invention to provide an improved impeller, a pressure generator using such an impeller, and a pressure generating system and method using such a pressure generator that overcomes the aforementioned shortcomings of conventional pressure support systems using conventional blower designs. It is a further object of the present invention to provide a pressure support system and method with an improved impeller configuration that provides a substantially constant pressure output over a range of flow rates typically associated with pressure support therapy. It is a further object of the present invention to provide a pressure support system and method with an improved impeller configuration that minimizes or eliminates the need for feedback control of the impeller in a pressure support system. A further object of the present invention is to provide an efficient blower and impeller design for use in respiratory support and other systems requiring a flow of gas.
These objects are achieved according to one embodiment of the present invention by providing a pressure generator that includes a housing having a gas inlet and a gas outlet, a motor, a rotatable drive shaft driven by the motor, and an impeller mounted on the drive shaft and disposed within the housing. The impeller includes a hub attached to the drive shaft, an impeller body attached to the hub extending radially from the hub to a perimeter of the impeller, and a plurality of impeller blades disposed on one face of the impeller body. Each impeller blade extends from a leading end of the blade, generally adjacent the hub, toward a trailing end of the blade, generally at the perimeter of the impeller. An inlet area is defined between each pair of adjacent blades generally adjacent the hub. Each inlet area is defined as the area at the radius of the leading end of the adjacent blades bounded by a height of the leading end of the adjacent blades and the one face of the impeller body. In addition, an outlet area is defined between each pair of adjacent blades generally adjacent the perimeter of the impeller. Each outlet area is defined as the area at the radius of the trailing end of the adjacent blades bounded by the height of the trailing end of the adjacent blades and the one face of the impeller body. Each inlet area is substantially equal to each corresponding outlet area for each pair of adjacent blades.
These objects are also achieved according to the present invention by providing a pressure support system and method of providing a flow of breathing gas that includes a source of breathing gas, a pressure generator, and a patient circuit coupled to the pressure generator for delivering the flow of breathing gas from the pressure generator to an airway of the patient. The pressure generator includes a motor, a rotatable drive shaft driven by the motor, and an impeller mounted on the drive shaft. The impeller is configured and arranged such that the pressure generator delivers a substantially constant pressure over a range of flows output from the pressure generator, including flows in the range of 10-150 l/min. In addition, the pressure generator delivers the substantially constant pressure selected over a range of pressures, including pressures in over a range of 10-65 cmH2O, which is established based on a rotational speed of the motor.